This invention relates to a high stability spin stand platform for testing read/write head assemblies used in computer hard disk drives.
Over the past decade, hard disk drive storage capacities have increased dramatically. This increase in storage capacity has resulted, in part, from the rapidly advancing technological developments in the magnetic sensitivity of the read/write head element. This rapid advancement in technology has made the read head very demanding to manufacture and as a result, virtually all of the read heads manufactured, and certainly all advanced technology heads, require acceptance testing.
The electrical characteristics of the read head are qualified on what is known in the industry as an electrical tester. This test device incorporates a motion platform which mimics the motions of the read head in an actual disk drive. The motion platform usually includes a coarse positioning stage and a micropositioning stage. The electrical tester also incorporates sophisticated electronics which test the read head element. Although the electrical qualification of the read head element has always been demanding, the motion requirements of the electrical tester have only recently pushed the limits of available technology.
As stated above, a two stage motion system typically consists of a coarse positioning stage and a micropositioning stage. In a computer disk drive, the data is stored along a spiral track on the disk. The radial spacing between tracks is as dense as 10,000 tracks per inch (tpi), currently, and as dense as 30,000 tpi is planned in the future. In order to qualify the performance of the read head, it is desirable to have the capability to move the head in discrete motion increments which are two orders of magnitude (10.sup.2) less than the track spacing. At 10,000 tpi, this is a motion increment of 1 .mu.inch (25 nanometers). These fine motions are performed by the micropositioning stage which typically includes a piezoelectric crystal which expands and contracts in response to an applied voltage. This device has a very limited range typically on the order of 10 .mu.m, so that it is necessary to reposition the entire micropositioning stage in order to test the read head at different locations on the disk. Most electrical tester configurations require a full range travel of 100 to 150 mm. In an actual disk drive, the read head traverses from the outer radius to the inner radius of the disk drive along an angular path (the head is mounted to a swing arm, similar to a record player) and thus, the relative angle of the head changes relative to the vector tangent to the servo tracks. This change in angle is termed the skew angle and variations in the skew angle have an effect on the performance of the head. In some electrical testers, however, the head is moved in a cartesian XY plane. It can be shown mathematically that the complete range of skew angles can be recreated by moving the head to various XY positions. This attribute makes these testers very flexible to different disk drive configurations in which the disk diameter and pivot angle of the drive arm vary.
A typical testing sequence for an electrical tester begins with the read head assembly, consisting of read heads mounted to a flexure arm, being mounted into a fixture (often referred to as the "nest") on a micropositioning stage. A spindle for supporting a magnetic disk is accelerated to an operating speed, typically 7,200 to 14,400 rpm. A coarse positioning stage moves the read head to a first test position and the read head is commanded to perform an adjacent track erase, in which all magnetic information on the disk is erased for the test track and for the adjacent inner and outer tracks. The read head is then commanded to write a stream of data to the disk of some finite length, typically less than a full rotation of the disk. The micropositioning stage then moves the head "off-track", usually to the position of what would be the next adjacent track to verify that the magnetic information from the test track cannot be sensed from the adjacent track. The micropositioning stage then moves the head across the test tract in discrete motion increments as small as 25 nm to correlate the intensity of the magnetic data on the test track to the radial position on the disk. In many newer heads in which the read and write heads are not co-located, this test also determines the relative spacing of the read and write elements. The coarse positioning stage is then moved to subsequent test points and the above steps are repeated. At the end of the test, the coarse positioning stage fully retracts to the load position and the read head assembly is exchanged for an untested assembly.
In order to optimize the total cycle time for the test sequence, it is highly desirable for the discrete motion increments of the micropositioning stage to be completed (both move time and time to settle within a stability band such as 10 nm) in less time than is required for a single rotation of the disk (e.g., 0.0043 seconds at 14,400 rpm). This goal is not currently achievable with existing designs.
The principle elements of prior art testers are a high speed air bearing spindle and a two stage motion system (i.e., a coarse positioning stage and a micropositioning stage). Existing tester designs typically utilize XY stacked stages, whether air bearing or mechanical. The stacked stages limit the capability of the micropositioning stage due to the dynamics of the stacked stage design. Even when an individual stage is made to be stiff, the ultimate structural dynamics for each stage are determined by the limitations of the drive actuator of the orthogonal stage axis. Such stages typically use leadscrew drives which have limited stiffness. Even the most rigid stacked stage designs have 1st mode natural frequencies of less than 200 Hz. Thus a single period of oscillation due to an external perturbation will take 0.005 seconds. Therefore, any positional perturbations due to the reaction forces from the micropositioning stage which exceed a stability band of 10 nm in magnitude make it impossible to achieve the goal of a sub-5 millisecond micropositioning move. In actual application, it is desirable for the coarse positioning stage to have a 1st mode natural frequency in excess of 1 kHz.
Due to the continually increasing storage density of disk drives, the testing of read/write heads currently in development will require levels of position stability that is difficult to achieve with current technology. In addition, constant price pressure due to a competitive marketplace is requiring increases in manufacturing throughput. As a result the spin stand platform now needs to move the read head under test more rapidly while at the same time providing a high level of positional stability. The typical stability requirement for the coarse positioning stage is to have no more than .+-.10 nm of variation in position over a period of several seconds. An additional requirement is that once positioned, the coarse stage must be extremely rigid so that reaction forces from the moving piezo stage and imbalance forces from the rotating spindle induce very little motion (in the form of vibration) in the coarse positioning stage. The relative position of the read head with respect to the spindle must remain stable within the .+-.10 nm band; thus any motion of the coarse positioning stage will compromise this level of performance.
The spin stand platform of the invention was conceived in order to provide several key advantages over existing spin stand platforms. A primary advantage is in the area of position stability under test. The spin stand platform of the invention can accommodate the stringent requirements resulting from ever denser radial spacing of tracks on computer disk drives.